1. Field of the Invention
The present invention relates to a hot-rolled steel wire rod having high drawability and capable of being drawn into a high strength fine steel wire with excellent corrosion resistance; a high strength fine steel wire obtained by drawing of the same steel wire rod and a twisted steel wire obtained by twisting of the same high strength steel wires; and a method of manufacturing the same fine steel wire.
2. Description of the Related Art
A fine steel wire used for a steel cord is generally manufactured in the following procedure: First, a steel material is hot-rolled and is subjected to controlled-cooling. Subsequently, the steel wire rod with a diameter of about 5.0 to 6.4 mm thus obtained is successively subjected to primary drawing, patenting, secondary drawing, re-patenting and brass plating, and then finally wet-drawn into a fine steel wire. A steel cord is manufactured by twisting of the fine steel wires thus obtained. In general, the fine steel wire has a diameter of about 0.35 to 0.175 mm. In the twisting process, several wires or several tens of wires of the fine steel wires are twisted into a steel cord.
In manufacture of such a steel cord, heavy reduction of area of 90 to 98% is applied in the wet-drawing process after brass plating, and further, a torsional stress and a tensile bending stress much stronger than the stress applied during the above wet-drawing process is applied in the subsequent twisting process.
Accordingly, the steel wire rod for the fine steel wire generally requires physical properties capable of preventing the breakage of wire in the subsequent drawing and twisting processes. In particular, for the above-described reason, it is important that the steel wire rod does not cause problems such as the breakage of wire in the wet-drawing process and the subsequent twisting process, and the seizure of a die in the drawing process when the steel wire rod is (to be) mechanically descaled.
In the usual drawing with the help of mechanical descaling, the thicker the scale, the better the drawability in the primary drawing, and accordingly, the hot-rolling conditions are determined so as to obtain thicker scales. However, this method to obtain thicker scale leads to an decrease in the yield ratio of the steel wire rod.
Fine steel wire with a high resistance to delamination is desired in the manufacture of steel cord with a tensile strength exceeding the value calculated by the equation TS=291-1275.times.Log.sub.10 D [TS(N/mm.sup.2): Tensile strength, D(mm): wire diameter of fine steel]. When the tensile strength of the fine steel wire exceeds the value calculated by the above equation, the frequency of delamination in the torsion test sharply increases. If the delamination occurs in the twisting process subsequent to the wet-drawing process, the lay length becomes uneven along the length of the steel cord, thus making it impossible to obtain the normal steel cord.
The steel grade frequently used at present time is SWRH82A prescribed in JIS G 3506. The fine steel wires made from this steel have tensile strengths of about 3400 N/mm.sup.2 at 0.2 mm dia., 3200 N/mm.sup.2 at 0.3 mm dia. These tensile strengths are set at less than the value obtained from the equation TS=2650-1275.times.Log.sub.10 D [TS(N/mm.sup.2): Tensile strength, D(mm): wire diameter of fine steel]. The present inventors have found that the use of a combination of a special wet-drawing method and addition of special elements into steel is highly effective in preventing delamination.